The gauge was maintained by conditional inc on "this sessionId isn't
in the in-memory `docs` Map yet" and dec on "last client for the
sessionId left." Because `docs` caches entries indefinitely — even
after every client disconnects — a reconnect found the doc still
cached, skipped the inc, then decremented again on the next
disconnect. Every reconnect-after-last-disconnect leaked one
decrement. After a day of normal reload / wifi-blip / tab-hibernate
patterns the gauge hit -182.
Replace the transitions with a `set()` derived from the live `conns`
map: count distinct session ids with at least one open WebSocket.
Self-correcting, no drift possible.
Extract `countActiveSessions()` as a pure helper + unit tests
including a regression case that simulates repeated reconnect/
disconnect cycles on one session and asserts the count never goes
negative.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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